The present invention relates to an angular rate sensor.
As a known angular rate sensor of the prior art, there is disclosed one in page 26 through page 33 of the Journal of Nippondenso Engineering Society (Vol. 38, No. 3, 1994). This angular rate sensor comprises an exciting unit for providing a tuning fork vibrator with vibration, a means for detecting a vibration level of the vibrator, a detection means for detecting Coriolis"" force generated responsive to an angular rate, a first amplifier for amplifying an output signal of the means for detecting vibration level, a rectifier circuit for rectifying an output signal of the first amplifier to obtain a DC voltage, a comparator of an output voltage of the rectifier circuit with a reference voltage, and a variable gain amplifier connected to the exciting unit in a manner that a vibrating amplitude of the tuning fork vibrator is controlled to be constant by varying an amplification factor for amplifying a voltage, which is produced by shifting a phase of an output voltage of the first amplifier by 90 degrees, according to an output voltage of the comparator.
With the prior art technique as described above, there is an effect to a certain extent for shortening a start-up time of the sensor, because the variable gain amplifier operates in a manner to rapidly increase a vibrating amplitude of the tuning fork vibrator, since the amplification factor of the variable gain amplifier becomes the maximum within a predetermined degree immediately after a power supply is turned on. However, because a variable range of the amplification factor of the variable gain amplifier is limited and it can not be boosted so much, it has needed a fairly long time for the tuning fork vibrator before a level of its vibrating amplitude reaches a predetermined degree. In addition, although it is possible to shorten the start-up time of the sensor if the maximum level of amplification factor of the variable gain amplifier is set greater than the predetermined degree, it is difficult to increase the maximum level of amplification factor of the variable gain amplifier beyond the predetermined degree from an overall aspect including a problem of saturation in waveform of an output voltage of the variable gain amplifier, maintaining noise low, and stability in controlling a vibrating amplitude of the tuning fork vibrator constant.
It is therefore difficult to shorten a time (the start-up time) required to bring the vibrating amplitude of the tuning fork vibrator constant without boosting the maximum level of amplification factor of the variable gain amplifier beyond the predetermined degree. Moreover, it is extremely difficult to reduce a time constant xcfx84=Q/2xcfx80f (where xe2x80x9cfxe2x80x9d is a driving frequency) without a substantial increase of xe2x80x9cfxe2x80x9d, in a such case as a crystal tuning fork vibrator having a large degree of sharpness xe2x80x9cQxe2x80x9d (vibrating energy/supplied energy). Therefore, it becomes a considerable obstacle to be not able to boost the maximum level of amplification factor of the variable gain amplifier beyond the predetermined degree in order to reduce the start-up time.
An angular rate sensor includes an exciting unit for providing a vibrator with vibration, a means for detecting a vibration level of the vibrator, a detection means for detecting Coriolis"" force generated responsive to an angular rate, a first amplifier for amplifying an output signal of the means of detecting vibration level, a rectifier circuit for rectifying an output signal of the first amplifier to obtain a DC voltage, a variable gain amplifier for taking as an input the output signal of the first amplifier and for varying an amplification factor according to an output voltage of the rectifier circuit, a second amplifier for amplifying an output signal of the variable gain amplifier, a level judgement circuit in which the output voltage of the rectifier circuit and an output voltage of a reference voltage generator are input, and a third amplifier and a switching means placed between the first amplifier and the second amplifier. The output signal of the first amplifier is input to a positive input terminal of the third amplifier. A first resistor is inserted between a negative input terminal of the third amplifier and an output terminal of the third amplifier. A voltage having a potential in the vicinity of xc2xd of a power supply voltage is impressed on the negative input terminal of the third amplifier. A second resistor and the switching means are connected in series between the output terminal of the third amplifier and an input terminal of the second amplifier. The switching means is activated by an output of the level judgement circuit.
A structure as described above is able to realize a reduction of the start-up time of the angular rate sensor, while clearing entirely the general matters such as a problem of saturation in waveform of an output voltage of the variable gain amplifier, maintaining noise low, and stability in controlling a vibrating amplitude of the tuning fork vibrator constant. It is especially effective in such case as a crystal tuning fork vibrator having a large degree of sharpness xe2x80x9cQxe2x80x9d.